The present invention relates to a stabilized microwave oscillator of the type including a rectangular waveguide operated below its cutoff frequency, associated with at least one active semiconductor element connected to a direct voltage supply, a resonator and a coupling arrangement which connects the resonator to the waveguide. A resonator of this type is disclosed in Federal Republic of Germany Pat. No. 2,805,254.
Microwave oscillators including semiconductor elements, such as Gunn or IMPATT [Impact Avalanche and Transit Time] oscillators are frequently stabilized in a passive manner.
The basic principle is to couple the active semiconductor element to a high quality resonator in such a manner that, with a given constant output power, the desired stability and simple tuning is realized over the desired frequency range, as explained in a paper by R. Knochel entitled "Design of Cavity-stablised Microwave Oscillators", published in Electronic Letters, London, Aug. 21, 1975, Volume 11, No. 17, at pages 405 and 406.
Structural measures keep the resonant frequencies of such high quality stablizing circuits independent of changes in temperature as well as humidity.
Three possibilities are known for interconnecting the main circuit including the active element with the stabilizing circuit (resonator).
Reaction oscillators (e.g. described in the above-cited paper in Electronic Letters, Volume 11, No. 17): they permit high external Q factors and relatively high output power. The Q factor of the main circuit is relatively high. The Locking and tuning range is relatively narrow and one-knob tuning over a given radio frequency range (.gtoreq.3% of the relative bandwidth) is difficult to realize. Additional suppression of the resonant frequency by the stabilizing circuit does not occur in the unlocked arrangement.
Reflection resonators (e.g. described in Federal Republic of Germany Pat. No. 2,805,254): they have a relatively large Locking and tuning range and relatively greater output powers. Their drawback is that they have a low external Q factor Q.sub.L. Since the main circuit also has only poor Q factors, temperature stabilization is more difficult, and there is no suppression of the resonant frequency when the oscillator is unlocked.
Transmission resonators (e.g. described in Federal Republic of Germany Pat. No. 2,356,445 and IEEE Proceedings Letters, 1970, Vol. 58, pages 831 and 832): due to their additional transmission losses, they produce less output power, with the same semiconductor element, than the reaction or reflection resonators. They have average external Q factors Q.sub.L. Their Locking and tuning range is good. Frequency changes (5 to 10% of the relative bandwidth) with single-knob tuning by detuning of a temperature compensated stabilization circuit can here be realized the easiest.
The resonant frequency of an unlocked oscillator should additionally be suppressed, if possible. In reaction and reflection resonators this may possibly be accomplished by additional monitoring and turn-off of the oscillator coupled with the monitoring so that the transmission of a wrong frequency is prevented in any case. This requires additional, often considerable, expenditures for circuitry.